clang/win: Let some chromium_code targets build with -Wextra.

chrome/browser/policy/policy_path_parser_win.cc

Index: chrome/browser/policy/policy_path_parser_win.cc
diff --git a/chrome/browser/policy/policy_path_parser_win.cc b/chrome/browser/policy/policy_path_parser_win.cc
index 3fa8a99475acccaa2dbe04338423d4b4c88dad55..2e0c197e1acacd0498df8fdadcef6a5cbfd1159b 100644
--- a/chrome/browser/policy/policy_path_parser_win.cc
+++ b/chrome/browser/policy/policy_path_parser_win.cc
@@ -77,7 +77,7 @@ base::FilePath::StringType ExpandPathVariables(
     result = result.substr(1, result.length() - 2);
   }
   // First translate all path variables we recognize.
-  for (int i = 0; i < arraysize(win_folder_mapping); ++i) {
+  for (size_t i = 0; i < arraysize(win_folder_mapping); ++i) {
     size_t position = result.find(win_folder_mapping[i].name);
     if (position != std::wstring::npos) {
       WCHAR path[MAX_PATH];